Photoactive devices are devices that are configured to convert electrical energy into electromagnetic radiation, or to convert electromagnetic radiation into electrical energy. Photoactive devices include, but are not limited to, light-emitting diodes (LEDs), semiconductor lasers, photodetectors, and solar cells. Such photoactive devices often include one or more planar layers of III-V semiconductor material. III-V semiconductor materials are materials that are predominantly comprised of one or more elements from group IIIA of the periodic table (B, Al, Ga, In, and Tl) and one or more elements from group VA of the periodic table (N, P, As, Sb, and Bi). The planar layers of III-V semiconductor material may be crystalline, and may comprise a single crystal of the III-V semiconductor material.
Layers of crystalline III-V semiconductor material generally include some quantity of defects within the crystal lattice of the III-V semiconductor material. These defects in the crystal structure may include, for example, point defects and line defects (e.g., threading dislocations). Such defects are detrimental to the performance of photoactive devices fabricated on or in the layer of III-V semiconductor material.
Additionally, currently known methods for fabricating layers of crystalline III-V semiconductor material generally involve epitaxial growth of the III-V semiconductor material on the surface of an underlying substrate, which has a crystal lattice similar to, but slightly different from the crystal lattice of the crystalline III-V semiconductor material. As a result, when the layer of crystalline III-V semiconductor material is grown over the different underlying substrate material, the crystal lattice of the crystalline III-V semiconductor material may be mechanically strained. As a result of this strain, as the thickness of the layer of III-V semiconductor material increases during growth, stress within the layer of III-V semiconductor material may increase until, at some critical thickness, defects, such as dislocations, become energetically favorable and form within the layer of III-V semiconductor material to alleviate the building stress therein.
In view of the above, it is difficult to fabricate relatively thick layers of crystalline III-V semiconductor material having relatively low concentrations of defects therein.
Photoactive devices may comprise an active region that includes a number of quantum well regions, each of which may comprise a layer of III-V semiconductor material. The quantum well regions may be separated from one another by barrier regions, which also may comprise a layer of III-V semiconductor material, but of different composition relative to the quantum well regions.
There is a discrepancy between the mobility of electrons and electron holes (vacant electron orbitals) in at least some III-V semiconductor materials. In other words, electrons may move through the III-V semiconductor materials relatively easier relative to electron holes. This discrepancy in the mobility between electrons and electron holes can lead to a non-uniform distribution of electrons and electron-holes within the active regions of photoactive devices. This phenomenon is discussed in further detail in X. Ni et al., Reduction of Efficiency Droop in InGaN Light Emitting Diodes by Coupled Quantum Wells, Applied Physics Letters, Vol. 93, pg. 171113 (2008), and in C. H. Wang et al., Efficiency Droop Alleviation in InGaN/GaN Light-Emitting Diodes by Graded-Thickness Multiple Quantum Wells, Applied Physics Letters, Vol. 97, pg. 181101 (2010), each of which is incorporated herein in its entirety by this reference.